1. Field of the Invention
The present invention relates to a projector which projects a video image onto a screen using a discharge lamp as a light source. More specifically, the present invention relates to a lamp lighting circuit which drives and controls the discharge lamp, and a method of controlling to light the lamp.
2. Description of the Related Art
A projector that uses a discharge lamp as the light source needs regular lamp replacement because the life of a lamp is incomparably shorter than other video displays. This is the most serious problem of the projector that uses a discharge lamp as the light source.
To solve this problem, lamp makers have invented all the possible device to prolong the life of discharge lamps. However, many constraints are imposed on the use of lamps because the luminous flux required is very great. That is, the performance required of a lamp can be displayed and the quality of the lamp as a product is ensured only when it is used under the constraints. The conditions under which the lamp is used include angles at which it is mounted, temperatures at which it is used, and driving power supply specifications.
The lighting of the lamp by regular power is also one of the basic constraints. The brightness adjustment based on power control as generally performed in illumination is basically impossible. Different regular power will require different optimum values of current and voltage at the start of the lamp and different timing of applying the current and voltage to the lamp. A lamp lighting circuit (this circuit is commonly known as “ballast”) involves the optimum design of control process to correspond to regular power of a used lamp.
Under such situations, there are demands for suppressing noise caused by a cooling fan even if the outgoing luminous flux is reduced and for reducing the lamp power for energy saving. In recent years, to meet such demands, a discharge lamp and its associated lighting circuit have been provided which can change the power of a lamp in steps within a limited range to effect brightness control. However, these lamp and lighting circuit are implemented at the cost of shortening the life of a lamp and degrading its burst and flicker resistance. These are left as problems to be solved in future.
Japanese Unexamined Patent Publication 2001-133882 discloses a projection display which is composed of a main block and a control block. The main block is adapted to apply power to a discharge lamp and the control block is adapted to control the main block in accordance with the characteristic of the discharge lamp. When the discharge lamp is replaced with another lamp of a different type, the control block is replaced correspondingly. Thus, different discharge lamp control characteristics can be obtained. In this example, however, it is required to replace a control block each time a discharge lamp is replaced with another type of discharge lamp. This is impractical.
Used in part of conventional projectors, the discharge lamp and the lamp lighting circuit that allow power to be switched within a limited range are used under the circumstance that the performance is degraded within an acceptable range. Thus, in the prior art, when a discharge lamp is replaced or when the lamp driving power is changed, the lamp cannot be driven at the optimum level of performance.
A projector is required to produce outgoing luminous flux most suitable for an environment in which it is mounted. The outgoing luminous flux is virtually determined by the selection of a light source lamp, an optical system, and a light bulb. Among them, it is the light source lamp that most greatly influences the outgoing luminous flux.
As the light source lamp, use is generally made of a discharge lamp typified by an extra-high pressure mercury lamp. With this type of discharge lamp, in order to increase the outgoing luminous flux, the arc length is shortened to closely approximate a point light source, the shape of a reflecting mirror is optimized, or the shape of an emission tube is optimized. Recently, however, these elements each have been optimized by respective individual makers and consequently the outgoing luminous flux has become dependent mainly on the lamp power. Thus, knowing the specifications of the light bulb and the optical system allows a rough outgoing luminous flux of the projector to be determined from the wattage of the lamp.
If, when a projector is actually used, there arises the need of lamp replacement, a demand may be made for reducing noise caused by the cooling fan instead of dropping the lamp power because the luminous intensity is enough. In general, the greater the lamp power, the shorter the life of the lamp becomes. Hence, there is also a demand for dropping the lamp power in order to give the life preference over the luminous intensity. Depending on the maker, a replacement lamp is generally costly and its price varies according to its power. For this reason, there is also a demand for purchasing a replacement lamp less expensive than the one used so far even if the luminous intensity is changed. If a difference in hue of projected video images resulting from a difference in lamp power or maker is not desirable, there is also demand for changing the lamp power or maker. However, the conventional techniques cannot meet these problems satisfactorily.